Funny, we were just talking about this from the point of view of the person outside the marriage: http://www.polyamory.com/forum/showt...038#post113038
I would say that it's certainly possible, and that the right prospective partner might even prefer it if they don't want to know your partner either or don't want to be out as poly, but I know that for me it wouldn't work. Like, if I got my girlfriend a silly little present, like a stuffed animal, I'm ok with her pretending to co-workers or extended family that it's not a romantic present, or that her husband gave it to her. But if she couldn't even display it openly in her home and had to, like, hide it in a box in the back of the closet because her husband wouldn't be comfortable seeing it, that would feel very depressing to me. That's just one random example of how I might find that sort of situation too limiting. Another example would be that I could never join her on a special day, like if she was in grad school and was having a big graduation ceremony, if her husband refused to meet me. I guess I could go and stand in the back in a trenchcoat, or just lie and pretend to be a friend. But still, so depressing.
But people do make it work. AnotherConfused is working under a different but related premise in her marriage about keeping things separate and has found happiness: http://www.polyamory.com/forum/showthread.php?t=16732
I think a lot depends on exactly how strict your husband needs the boundaries to be.